I wonder why he chose Trabzonspor when there are other elite clubs from the Turkish league such as Galatasaray or Fenerbahce, who compete in Europan competitions while Trabzonspor only competes in the local league, is it only Trabzonspor who made an offer? or maybe this is also related to salary I mean only Trabzonspor are willing to pay ??
Maybe he just wanted a club where he could be away from the pressure of performance so he can rebuild his confidence. He must be really damaged from Manchester United because his errors was amplified a lot and he was at the receiving end of many backlash from both the media and supporters in Manchester United. A break from all that attention can be good. He is not a bad goalkeeper, he just found himself in a system that did not favor him.
The transfer is more exciting for the supporters of Manchester United who did not hide their celebration, they have long been waiting for the move.
Not many clubs were interested in Onana, and even if there are, almost none will make an offer to Man United. For example, Inter Milan is rumored to be planning a reunion with Onana, but until the transfer window ended, Inter Milan never made an offer. I think Trabzonspor was the only club that made an offer to Man United, and it happened right before the transfer window closed, so everything had to be done quickly.
It's quite reasonable why not many clubs are interested because Onana's performance has been quite bad so far and he is known for making frequent blunders.